Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
|A computer montage of Jupiter and its four large (Galilean) satellites.|
|Images of the four Galilean satellites to the same scale. From upper left they are:
|This is what everyone expected the Galilean Satellites to look like - bland, heavily cratered icy balls. In fact, Callisto is the most heavily cratered body in the solar system. This is about the last thing Voyager showed that matched anyone's preconceptions!|
|Callisto has the champion multiple-ring impact basin in the solar system. Called Valhalla, it has about 30 distinct rings.
Like many bodies in the outer solar system, Callisto is covered with a dark coating, literally as dark as soot. It's not fair - icy objects orbiting where the sunlight is dim ought to be bright - but that's how it is. The coating is so sooty that most planetary geologists think that's just what it is - not from combustion, but from solar ultraviolet and atomic particles stripping hydrogen atoms off organic molecules and leaving carbon. Why this stuff coats some objects but not others is a deep mystery.
|Ganymede is the largest satellite in the solar system, beating Saturn's moon Titan by 120 km. This is a photomosaic (I have actually had people ask why Ganymede had jagged edges!).
Ganymede's icy crust seems to have fractured and drifted apart. Ganymede and Earth are the only two bodies where very large horizontal crustal movements have occurred, though the mechanisms are totally different. Ganymede does not have plate tectonics; its crustal movements are more akin to drifting of ice pack in the Arctic. Early in its history, Ganymede probably had an icy crust and a liquid interior.
Like Callisto, Ganymede has been darkened, and the darkening increases with age. Younger craters and crustal areas are lighter.
|Europa is covered with something that looks for all the world like the canal network Percival Lowell thought he could see on Mars. It's also almost crater-free, suggesting that something obliterates craters quickly.|
|The fissures on Europa look so much like aerial views of polar pack ice on Earth that most geologists have assumed that's exactly what we see: a thin icy crust with liquid water beneath.|
|The press and scientific community went ga-ga over Galileo images like this, which show features a kilometer or smaller in size and which look for all the world like Antarctic icebergs trapped in pack ice. To me, ever since Voyager, it's been more or less trivially obvious that that's what the surface must be like. I would much rather see a complete map of Ganymede so we could unravel its crustal movements.|
|Parts of Europa have smooth ice plains with healed fractures.|
|This is more nearly the natural color of Io
The large black horseshoe is Loki Patera, a hot spot in infrared images and apparently a lake of molten sulfur with a solid sulfur raft floating in it.
|Measuring 260 by 146 by 133 km, Amalthea was the only one of Jupiter's small moons to be photographed in any detail. It is about as big as an object can be before gravity deforms it into a round shape. One very large crater can be seen. The red coating is sulfur that escaped from Io.|
Created 6 April 1999, Last Update 14 September 2018